hull vibration

[manon]

Good day everyone.
I would like to know the possible reasons of vibration of a 30 meter length Pilot vessel. Initially, after built, there was no problem with vibration, but there was some problem in stability during turing. so the vessel was modified with some solid balast in order to gain the turing stability, but now she has excessive vibration. the vibration is very high.
what could be the possible reasons?
Well, I suppose, due to the change in draft and weight distribution, the wake field has changed, but is this change causing the excessive vibration?

Thanks in advance

[Ad Hoc]

It would be far better if you could provide some readings/values and where they were taken and at what ship conditions/headings/sea state etc and what kind of vibration is felt.

However, vibration or rather frequncey of vibration is directly proportional to the mass. So it seems, but can't say for sure without hard data, that where you have placed the solid ballast has made things worse. But without knowing the frequncey of excitation very hard to say any more...

[keysdisease]

What else was changed along with the ballast? Has the propeller been checked? Props are the first place I would look when a mystery vibration suddenly "appears."

A vibration analysis could probably tell you exactly where the source is. Vibrations like the one in your limited description are seldom caused by the hull, but are more commonly from some type machinery. Prop, prop/strut relationship, shaft, generator, gearbox, etc.

You may hear, "we didn't hit anything, it can't be the prop." Props are damaged all the time and Captains never hit anything, but somehow the props get damaged.

Steve

[daiquiri]

At the cost of being boringly repetitive, may I ask you to post few pics or drawings of your boat, and possibly one where the modificated parts are visible?

[baeckmo]

The ballast is fitted inboard, I assume or.....? Any modifications to hull lines involved (such as brackets or doublings on the "wet side")?

[manon]

12 tonnes solid ballast used at amidships, inboard. 200mm draft increased.
Loa=29.42m
Beam=6.1m
D=3.5m
d=2 m (final loaded)
Propeller dia=1298 mm
shaft CL=900 mm above baseline. no modification of the hull. I will try to upload lines, GA ASAP.
thanks for your responses

[daiquiri]

12 tonnes ballast concentrated amidships is quite a strain for the structure. It sounds like, say, 10-15% of the original vessel's displacement. I hope you have had all the structural calculations done properly before performing this modification.

Guesswork is a bad troubleshooting practice, but here is my 2 cents worth - with the limited info given so far: I say that the hull might have sagged under this concentrated weight, causing a distorsion and misalignment of the propeller shaft. You should check the shaft bearings for any sign of recent wear, which would be an indicator of a non-aligned shaft.

[viking north]

I'd say you might have hit the nail on the head--Possibly to the point where there might be some very minute whip and if testing the frequency don't let the frequency counter throw you off, the hull could be resonating at it's natural cycle which could be the second harmonic of the oscillating shaft or whatever the source. Get the freq. of the offending vibrations and if you fail to find a source corresponding then look for sources developing frequencies divisional by a factor of two. It is unlikely but possible that the weaker third or fourth harmonics could be the triggers.

[daiquiri]

Any further news regarding this story, Manon?

[viking north]

Guess Manon is busy unloading 12 tons of bagged sand

[Fanie]

Originally Posted by manon

Good day everyone.
I would like to know the possible reasons of vibration of a 30 meter length Pilot vessel. Initially, after built, there was no problem with vibration, but there was some problem in stability during turing. so the vessel was modified with some solid balast in order to gain the turing stability, but now she has excessive vibration. the vibration is very high.
what could be the possible reasons?
Well, I suppose, due to the change in draft and weight distribution, the wake field has changed, but is this change causing the excessive vibration?

Thanks in advance

Cheeeese mate, don't know what you're complaining about. That boat of your's should be a hit with the chicks ! Perhaps you're using it for the wrong purpose

Oh the problem. Perhaps from a motor... ?

[Milehog]

Originally Posted by Fanie

Cheeeese mate, don't know what you're complaining about. That boat of your's should be a hit with the chicks ! Perhaps you're using it for the wrong purpose

Perhaps someone installed one of these on the boat?

[Fanie]

Why go for somethin so small and that would draw extra battery power if you can have the WHOLE BOAT working for you ? No escape...